问javaScript中的setTimeout无法正常工作

EN

setTimeout(function(){
alert('hello');
} , 3000);

function functionName() {
    setTimeout(function(){ //Your Code }, 3000);
}

// timer gives us an empty named "placeholder" we can use later
// to reference the setTimeout instance. This is important because
// remember, JS is async. As such, setTimeout can still have methods
// conditionally set to work against it.

let timer 

// "delayHandler", defined below, is a function expression which when
// invoked, sets the setTimeout function to the empty "timer" variable
// we defined previously. Setting the callback function as the function 
// expression used to encapsulate setTimeout provides extendable control
// for us later on however we may need it. The "n" argument isn't necessary,
// but does provide a nice way in which to set the delay time programmatically
// rather than hard-coding the delay in ms directly in the setTimeout function
// itself.

const delayHandler = n => timer = setTimeout(delayHandler, n)

const ParentFunc = step => {

  // "Private" function expression for click button event handler.
  // Takes only one argument, "step", a.k.a the index
  // value provided later in our for loop. Since we want "clickButton"
  // to act as the callback to "clickDate", we add the "delayHandler"
  // method we created previously in this function expression.
  // Doing so ensures that during the for loop, "clickDate" is invoked
  // when after, internally, the "clickButton" method is fired as a
  // callback. This process of "Bubbling" up from our callback to the parent
  // function ensures the desired timing invocation of our "delayHandler"
  // method. It's important to note that even though we are getting lost
  // in a bit of "callback hell" here, because we globally referenced 
  // "delayHandler" to the empty "timer" variable we still have control
  // over its conditional async behavior.

  const clickButton = step => {
    console.log(step)
    delayHandler(8000)
  }

  // "Private" function expression for click date event handler
  // that takes two arguments. The first is "step", a.k.a the index
  // value provided later in our for loop. The second is "cb", a.k.a
  // a reference to the function expression we defined above as the
  // button click event handler method.

  const clickDate = (step, cb) => {
    console.log(step)
    cb(delayHandler(8000))
  }

  // Return our "Private" methods as the default public return value
  // of "ParentFunc"

  return clickDate(step, clickButton(step))
}

 for(let i = 0; i < 4; i++) {

   // Within the for loop, wrap "ParentFunc" in the conditional logic desired
   // to stop the setTimeOut function from running further. i.e. if "i" is 
   // greater than or equal to 2. The time in ms the setTimeOut was set to run
   // for will no longer hold true so long as the conditional we want defined
   // ever returns true. To stop the setTimeOut method correctly, use the 
   // "clearTimeout" method; passing in "timer", a.k.a our variable reference 
   // to the setTimeOut instance, as the single argument needed to do so.   
   // Thus utilizing JavaScript's inherit async behavior in a "pseudo" 
   // synchronous way.

   if(i >= 2) clearTimeout(timer)
   ParentFunc(i)
 }